Japan plans to avoid expressing direct support for actual U.S. military action against Syria, in an attempt to avoid damaging negotiations with Russia over a decades-long territorial row, government sources said Thursday.

Japan will only support U.S. President Donald Trump's "resolve" to prevent any further use of chemical weapons in Syria, the sources said.

Russia is a major backer of the government of Syrian President Bashar al-Assad in the country's civil war, and strongly opposes the U.S. claim that Syrian government forces attacked a rebel-held town near Damascus on Saturday with chemical weapons.

Having pledged to put an end to the dispute over the Russian-held, Japan-claimed chain of islands, Prime Minister Shinzo Abe hopes to make progress on the issue in talks with President Vladimir Putin during a visit to Russia next month.

Before that, Abe will meet Trump in the United States next week, with the aim of getting Japan's interests on the agenda of a proposed meeting in May or June between Trump and North Korean leader Kim Jong Un.

Trump posted a tweet on Wednesday warning Russia to "get ready" because missiles "will be coming" to Syria. Then Thursday he tweeted that an attack on Syria "could be very soon or not so soon at all!"

According to the sources, the specific response being considered in the event of U.S. military action against Syria is Japan's "support for the U.S. resolve to fulfill its responsibility to prevent the use of chemical weapons."

Tokyo issued a similar response in April last year when the United States conducted a missile strike on a Syrian air base that was the alleged source of a chemical attack that killed dozens of civilians.

"Japan cannot make a judgment on whether Assad's government forces really used chemical weapons," a senior Foreign Ministry official said of the latest attack.

Japan has also refrained from joining the United States and other countries in alleging Russia was responsible for the nerve agent poisoning of a former Russian spy in Britain last month.